The Cell type used with the TreeView control. In addition to
the API defined on IndexedCell, the TreeCell
exposes additional states and pseudo classes for use by CSS.

A TreeCell watches the selection model of the TreeView for which it is
associated, ensuring that it visually indicates to the user whether it is
selected. When a TreeCell is selected, this is exposed both via the
selected property, as well as via the 'selected'
CSS pseudo class state.

Due to the fact that TreeCell extends from IndexedCell, each TreeCell
also provides an index property. The index will be
updated as cells are expanded and collapsed, and therefore should be
considered a view index rather than a model index.

Finally, each TreeCell also has a reference back to the TreeView that it is
being used with. Each TreeCell belongs to one and only one TreeView.

setDisclosureNode

The node to use as the "disclosure" triangle, or toggle, used for
expanding and collapsing items. This is only used in the case of
an item in the tree which contains child items. If not specified, the
TreeCell's Skin implementation is responsible for providing a default
disclosure node.